This invention relates to a glaze composition for enhancing the surface smoothness of, for example, a ceramic substrate, and particularly to a glaze composition suitable for the manufacture of thin-film hybrid parts using thin-film or thin/thick-film techniques, such as chip capacitors, chip resistors and chip inductors.
Conventionally, a glaze composition is known as the composition for remarkably enhancing the surface smoothness of a ceramic substrate. The glaze composition is provided with heat accumulating, electric insulating and other superior vitreous properties. These properties are utilized by thermal heads and various other parts.
(1) The glaze composition, superior in surface smoothness, for thermal heads generally included an alkaline metal and lead, which raises the manufacturing efficiency of the glaze composition.
(2) Recently, a glaze composition for a high-speed or photographic printing thermal head was composed mainly of thermally resistant silicon dioxide, without including an alkaline metal or lead. Such a high-temperature glaze composition was disclosed in, for example, Japanese Examined and Published Patent Application Nos. 60-55453 and 61-24345, and Japanese Laid-open Patent Application No. 60-118648.
Nowadays glaze compositions are developed for use in chips in addition to the thermal heads. To meet the demands for light, thin, short, small but highly-precise electronic parts, a ceramic substrate with a glaze layer formed on the surface thereof is replacing conventional chip substrates. This glazed substrate has a superior surface smoothness, and thin or thick film patterns can be formed on the surface of the substrate with fine pattern pitches.
If the aforementioned conventional glaze composition (1), containing a large amount of alkaline metal and lead is used for a chip substrate, at the time of the application of voltage the alkaline metal decreases the electric insulation while moving through glass, thereby deteriorating electric reliability. When the glaze composition is burnt in a reducing atmosphere, the reduction of lead oxides turn glass black and the high electric insulation, i.e. a specific volumetric resistance of 10.sup.14 .OMEGA..cm or more, cannot be maintained. In addition, the use of lead is undesirable because lead is a toxic substance.
The aforementioned high-temperature glaze composition (2) mainly composed of silicate was developed so as to pursue the high heat resistance. If the thermal expansion coefficient of the composition fails to coincide accurately with that of the ceramic substrate, the ceramic substrate would warp. Since the high-melting glass requires a high fixing temperature when formed integrally on the substrate, even a slight difference of the glass from the substrate in the thermal expansion coefficient would cause the substrate to warp. Since the ceramic substrate requires a sufficient thickness for withstanding such warping, a desirably thin chip substrate is difficult to be formed.
At the time of glaze burning, the high-melting glass develops such a high viscosity that the end portions of glaze composition become remarkably protuberant relative to the entire thin glaze composition. This is not disadvantageous for partial glazing, but provides insufficient smoothness to the entire surface cover or band-shaped glaze patterns to be formed on the chip substrate.
The thermal resistance of the glazed substrate is an essential property for thermal heads, but is an unnecessarily quality for thin-film hybrid parts. The glazed substrate for the hybrid parts needs to be calcined in high temperatures. Therefore, a glazed substrate having a high thermal resistance is uneconomical for use of the thin-film hybrid parts.
The desired glaze thickness of the glazed substrate for thin-film hybrid parts is between about 5 .mu.m and about 25 .mu.m, while that of the conventional glazed substrate for facsimile thermal heads is between 60 .mu.m and 80 .mu.m. When a conventional glaze composition is made thin and the glazing viscosity is high, the composition is influenced by the irregular surface of the ceramic substrate. Therefore, the glaze composition provides insufficient surface smoothness. Such deterious change of the glaze face is undesirable. These days a glaze composition which can provide a very thin and flat but high-quality glaze face is demanded.